Non-ionic surfactants are widely used in commercial and household applications where advantage is taken of their superior performance as a wetting agent, their detergency and scouring characteristics, their adaptability for being combined with other types of surfactants and resistance to hard water conditions. Often such surfactants foam too much for certain applications, for example, in automatic dishwashers.
In recent years a number of non-ionic surfactants have been developed and used commercially, which are designated as low-foaming. When the need for foam suppression is of prime importance, the non-ionic surfactants developed to meet this requirement often have been found to sacrifice other desirable characteristics, such as detergency.
It has long been known to prepare non-ionic surfactants by the addition of ethylene oxide or mixtures of ethylene oxide and propylene oxide to various alcohols, generally long chain monohydric alcohols. Numerous different adducts have been described in the patent literature, some of which contain only oxyethylene groups while others contain a random distribution of oxyethylene and oxypropylene groups or discrete blocks of polyoxyethylene and polyoxypropylene. Representative prior art patents describing such adducts include U.S. Pat. Nos. 2,674,619, 2,677,700, 3,101,374, 3,770,701, 3,956,401, 4,410,477 and 4,411,810.
U.S. Pat. No. 3,956,401 describes a low foaming, biodegradable non-ionic surfactant which is liquid at room temperature and non-gelling in water solution. The surfactant has the formula: ##STR1## wherein R is a substantially linear alkyl group containing 7 to 10 carbon atoms, R, and R" are most desirably a methyl group, x is an integer of 1 to 6, y is an integer of 4 to 15 and z is an integer of 4 to 25. This detergent, therefore, comprises successively an oxypropylene (PO) block, an oxyethylene (EO) block and an oxypropylene (PO) block adducted to the alcohol.
U.S. Pat. No. 4,410,447 also describes a liquid low-foaming non-ionic surfactant, which is also said to have good scouring and/or detergency. This surfactant has the formula: EQU R-O-Ax-B-H
wherein R is a primary alkyl group having from 7 to 11 carbon atoms, A is a block of oxypropylene groups, x is an integer from 2 to 15 with the proviso that the total number of carbon atoms in R and A is at least 12, and B is a random mixture of oxyethylene groups and oxypropylene groups wherein the molar ratio of oxyethylene to oxypropylene in A and B is from 0.2:1 to 1.5:1 while the molar ratio of oxyethylene to oxypropylene in B is from 1:1 to 5:1. The compositions have a cloud point from about 20 to about 60.degree. C. This prior art patent, therefore, discloses a detergent which comprises successively a PO block and a random mix of EO-PO adducted to the alcohol.
It is stated in this prior art patent that only by employing alcohols having a particular combination of carbon atoms and various essential amounts of alcohol, propylene oxide and mixtures of ethylene oxide and propylene oxide can products be prepared which are liquid and low foaming and have superior wetting properties.
U.S. Pat. No. 4,411,810 to Dutton et al describes a low foaming non-ionic polyoxyalkylene surfactant for incorporation into dishwashing formulations, so that alkyl phosphate ester defoaming agents can be eliminated from the formulations. The useful polyoxyalkylene surfactants are described as having the formulas: ##STR2## wherein EO represents ethylene oxide, Y represents the nucleus of an active hydrogen-containing organic compound having a functionality x and (1) about 2 to about 6 aliphatic carbon atoms and 2 to 3 reactive hydrogen atoms or (2) about 6 to about 18 aliphatic carbon atoms and 1 to 3 reactive hydrogen atoms, A represents a lower alkylene oxide including propylene oxide, EO/A represents a mixture of ethylene oxide and a lower alkylene oxide in which EO/A are present in the proportions by weight of 5 to 95 to 5 percent, and m, n and o are integers individually selected such that the polymer has an average total molecular weight of about 500 to about 25,000. This prior art patent, therefore, describes a very wide range of surfactants useful in the dishwashing detergent compositions. Of interest to the present invention are compounds corresponding to formula I, which comprise a random mixture of EO and PO and a PO block adducted to the alcohol. The surfactants are said to require a cloud point in a 1 wt. % aqueous solution of about 15.degree. to about 25.degree. C. in order to be effective.
Although this reference describes the broad classes of widely-varying molecular weight polyoxyalkylene surfactants as low foaming, it will be apparent from the succeeding description of the invention that there are several critical criteria which must be met to provide a combination of low foaming and good detergency.